In medical radiography an image of a patient's tissue and bone structure is produced by exposing the patient to X-radiation and recording the pattern of penetrating X-radiation using a radiographic element containing at least one radiation-sensitive silver halide emulsion layer coated on a transparent (usually blue tinted) film support. The X-radiation can be directly recorded by the emulsion layer where only low levels of exposure are required, as in dental imaging and the imaging of body extremities. However, a more efficient approach, which greatly reduces X-radiation levels required, is to employ an intensifying screen in combination with the radiographic element. The intensifying screen absorbs X-radiation and emits longer wavelength electromagnetic radiation which silver halide emulsions more readily absorb. Another technique for reducing patient exposure is to coat two silver halide emulsion layers on opposite sides of the film support to form a "double coated" radiographic element.
Diagnostic needs can be satisfied at the lowest patient X-radiation exposure levels by employing a double coated radiographic element in combination with a pair of intensifying screens. The silver halide emulsion layer unit on each side of the support directly absorbs about 1 to 2 percent of incident X-radiation. The front screen, the screen nearest the X-radiation source, absorbs a much higher percentage of X-radiation, but still transmits sufficient X-radiation to expose the back screen, the screen farthest from the X-radiation source. In the overwhelming majority of applications the front and back screens are balanced so that each absorbs about the same proportion of the total X-radiation. However, a few variations have been reported from time to time. A specific example of balancing front and back screens to maximize image sharpness is provided by Luckey et al U.S. Pat. No. 4,710,637. Lyons et al U.S. Pat. No. 4,707,435 discloses in Example 10 the combination of two proprietary screens, Trimax 2.TM. employed as a front screen and Trimax 12F.TM. employed as a back screen. K. Rossman and G. Sanderson, "Validity of the Modulation Transfer Function of Radiographic Screen-Film Systems Measured by the Slit Method", Phys. Med. Biol., 1968, vol. 13, no. 2, pp. 250-268, report the use of unsymmetrical screen-film assemblies in which either the two screens had measurably different optical characteristics or the two emulsions had measurably different optical properties.
An imagewise exposed double coated radiographic element contains a latent image in each of the two silver halide emulsion units on opposite sides of the film support. Processing converts the latent images to silver images and concurrently fixes out undeveloped silver halide, rendering the film light insensitive. When the film is mounted on a view box, the two superimposed silver images on opposite sides of the support are seen as a single image against a white, illuminated background.
It has been a continuing objective of medical radiography to maximize the information content of the diagnostic image while minimizing patient exposure to X-radiation. In 1918 the Eastman Kodak Company introduced the first medical radiographic product that was double coated, and the Patterson Screen Company that same year introduced a matched intensifying screen pair for that product.
An art recognized difficulty with employing double coated radiographic elements in combination with intensifying screens as described above is that some light emitted by each screen passes through the transparent film support to expose the silver halide emulsion layer unit on the opposite side of the support to light. The light emitted by a screen that exposes the emulsion layer unit on the opposite side of the support reduces image sharpness. The effect is referred to in the art as crossover.
A variety of approaches have been suggested to reduce crossover, as illustrated by Research Disclosure, Vol. 184, August 1979, Item 18431, Section V. Cross-Over Exposure Control. Research Disclosure is published by Kenneth Mason Publications, Ltd., Dudley Annex, 21a North Street, Emsworth, Hampshire P010 7DQ, England. While some of these approaches are capable of entirely eliminating crossover, they either interfere with (typically entirely prevent) concurrent viewing of the superimposed silver images on opposite sides of the support as a single image, require separation and tedious manual reregistration of the silver images in the course of eliminating the crossover reduction medium, or significantly desensitize the silver halide emulsion. As a result, none of these crossover reduction approaches have come into common usage in the radiographic art. An example of a recent crossover cure teaching of this type is Bollen et al European published patent application 276,497, which interposes a reflective support between the emulsion layer units during imaging.
The most successful approach to crossover reduction yet realized by the art consistent with viewing the superimposed silver images through a transparent film support without manual registration of images has been to employ double coated radiographic elements containing spectrally sensitized high aspect ratio tabular grain emulsions or thin intermediate aspect ratio tabular grain emulsions, illustrated by Abbott et al U.S. Pat. No. 4,425,425 and 4,425,426, respectively. Whereas radiographic elements typically exhibited crossover levels of at least 25 percent prior to Abbott et al, Abbott et al provide examples of crossover reductions in the 15 to 22 percent range.
Still more recently Dickerson et al (I) U.S. Pat. No. 4,803,150 has demonstrated that by combining the teachings of Abbott et al with a processing solution decolorizable microcrystalline dye located between at least one of the emulsion layer units and the transparent film support less than 10 percent down to "zero" crossover levels can be realized. Since the technique used to determine crossover, single screen exposure of a double coated radiographic element, cannot distinguish between exposure of the emulsion layer unit on the side of the support remote from the screen caused by crossover and the exposure caused by direct absorption of X-radiation, "zero" crossover radiographic elements in reality embrace radiographic elements with a measured crossover (including direct X-ray absorption) of less than about 5 percent.
Dickerson et al (II) U.S. Ser. No. 217,727, filed July 8, 1988, now U.S. Pat. No. 4,900,652, adds to the teachings of Dickerson et al (I), cited above, specific selections of hydrophilic colloid coating coverages in the emulsion and dye containing layers to allow the "zero" crossover radiographic elements to emerge dry to the touch from a conventional rapid access processor in less than 90 seconds with the crossover reducing microcrystalline dye decolorized.
An art accepted measure of the imaging efficiency of a radiographic element is its detective quantum efficiency (DQE). DQE is the ratio of the noise introduced into the radiographic element due to spatial fluctuations in the incident X-radiation supplied on exposure and the noise exhibited by the image bearing radiographic element, which is a composite of the input noise and the noise generated by the radiographic element itself. A theoretically perfect radiographic element is one which contributes no additional noise to that received on exposure and therefore exhibits a DQE of unity. In practice all radiographic elements exhibit a DQE substantially less than unity.
The DQE of a radiographic element can be determined by making two direct noise measurements, measurement of the input noise power spectrum (NPSi) and measurement of the output noise power spectrum (NPSo), and by making a mathematical adjustment to account for the influence of system gain (image contrast) and image sharpness as a function of its spatial frequency (modulation transfer factor or MTF).
DQE can be expressed mathematically by the following equation: ##EQU1## where
NPSi is the input noise power spectrum,
NPSo is the output noise power spectrum,
.gamma. is contrast, and
MTF is the modulation transfer factor at the spatial frequency of imaging being measured.
From equation E-I it is apparent that DQE is a dimensionless ratio of the input and output noise power spectra with adjustments for MTF and contrast, the latter being the ratio of density change (.DELTA.D) per log unit of exposure change (.DELTA. log E, where E is exposure in meter-candle-seconds). When contrast is 1.0, contrast ceases to be a significant factor in DQE. However, MTF is always a significant factor reducing DQE, since MTF is always less than unity when any degree of imaging detail is being considered. MTF is unity only when the spatial frequency of the image is 0-i.e., there is no image detail present. MTF typically declines from unity to a small fraction over the image spatial frequency range of 0 to 10 cycles/mm. Stated qualitatively, the image noise introduced by the radiographic element increases progressively as finer imaging detail is considered.
While equation E-I has been presented for ease of visualization, the more common practice is to employ the term incident X-radiation fluence (Q) instead of NPSi, where
(E-II) EQU Q=(log.sub.10 e).sup.2 /NPSi
(log.sub.10 e) is 0.4343 and Q has the units quanta/mm.sup.2. With the substitution of Q for NPSi, equation E-I takes the following form: ##EQU2##
One of the visualization advantages of employing Q in preference to NPSi flows from the fact that Q is proportional to E. Therefore, in the pilots below of log Q versus spatial frequencies in cycles per mm similar DQE profiles are obtained whether log Q or log E is employed as an ordinate.
DQE and its component terms are described and their use demonstrated by R. Shaw, "The Equivalent Quantum Efficiency of the Photographic Process", J. Photogr. Sci., 11, 199-204 (1963) and J. Dainty and R. Shaw, Image Science, Academic Press, London, 1976, especially pp. 152-189 and 276-319.